The present invention relates to a low pressure casting machine for casting copper alloy and, in particular, to a low pressure casting machine suitable for brass castings which are small in size and large in the number of lots.
In a conventional low pressure casting machine, a crucible for storing molten copper alloy such as molten brass is accommodated in a crucible chamber consisting of a lid and a chamber body which are airtightly connected to each other. The lid includes a molten copper alloy supply opening which can be airtightly opened or closed for receiving molten copper alloy from a melting furnace, and a pipe-shaped stoke which extends through the lid and rises from the bottom portion of the crucible. The stoke includes on the top end portion thereof a joint which can be connected with a metal mold. To the lid is connected a common pipe which connects, in parallel, a pressure pipe for applying pressure to the crucible chamber to thereby pour the molten copper alloy into the metal mold with an exhaust pipe for exhausting the air from the crucible chamber to thereby stop the pouring of the molten copper alloy. Further, the conventional crucible does not include any induction heating device but the above-mentioned molten copper alloy is kept hot by a heat insulator.
To pour the molten copper alloy into the metal mold, the molten copper alloy at suitable temperatures is poured from the melting furnace through the molten copper alloy supply opening into the crucible to the brim. The supply opening is then closed airtightly, and the metal mold is put on the joint disposed on the top end portion of the stoke. If the crucible chamber is given pressure through the common pipe from a gas pressure source, then, due to the principle of a siphon, the molten copper alloy moves upwardly through the stoke by the pressure that is applied onto the surface of the molten copper alloy within the crucible, so that the molten copper alloy is poured into the metal mold. After completion of pouring of the molten copper alloy, if the common pipe is opened to the atmosphere through the exhaust pipe, then the crucible chamber is returned to the atmospheric pressure so that the molten copper alloy in the stoke moves back into the crucible. Then, the metal mold is replaced with a new mold and, similarly, the pressuring and exhausting operations are performed repeatedly to thereby pour the molten copper alloy into a plurality of metal molds sequentially. As the surface of the molten copper alloy in the crucible is lowered, the pressuring forces are increased sequentially. If the molten copper alloy in the crucible is decreased, or if the temperature of the molten copper alloy reaches the lower limit of a range of temperatures suitable for casting, then the molten copper alloy at suitable temperatures is poured again from the melting furnace through the supply opening into the crucible.
In the above-mentioned conventional low pressure casting machine, when the metal to be casted is an aluminum alloy or brass, there can be obtained a wide range of casting temperature having allowable values, including the upper and lower limit values of temperatures suitable for casting of the molten metal. Likewise, when a metal mold is large in size and a piece of casting is large in weight, the molten metal poured from the melting furnace into the crucible to the brim is used out before the temperature of the molten metal in the crucible is lowered down to the lower limit of the casting temperature allowable values. Therefore, the low pressure casting machine using a metal mold provides a good operation efficiency. However, when the metal mold is small in size and a piece of casting is small in weight, because the molten metal pouring tact is not shortened so much, the temperature of the molten metal poured into the crucible to the brim is lowered down to the lower limit of the casting temperature allowable values before the molten metal is depleted. In view of this, it is necessary to replenish again the molten metal at high temperatures from the melting furnace while the previously poured molten metal is left half in the crucible, which requires an additional time. That is, the loss of time for the replenishing operation worsens the operation efficiency of the low pressure casting machine.
Conventionally, water service items such as a water tap and the like were originally made of bronze. In recent years, however in addition to the bronze, use of brass has also been permitted. In metallurgy, bronze is not suitable for casting by a low pressure casting machine using a metal mold and, for this reason, there is seen a tendency to produce a small-sized brass casting by use of a low pressure casting machine using a metal mold with high efficiency.